selftests/bpf: Add more tests for check_max_stack_depth bug

Another test which now exercies the path of the verifier where it will
explore call chains rooted at the async callback. Without the prior
fixes, this program loads successfully, which is incorrect.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230717161530.1238-4-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/async_stack_depth.c

@@ -22,9 +22,16 @@ static int timer_cb(void *map, int *key, struct bpf_timer *timer)
 	return buf[69];
 }
 
+__attribute__((noinline))
+static int bad_timer_cb(void *map, int *key, struct bpf_timer *timer)
+{
+	volatile char buf[300] = {};
+	return buf[255] + timer_cb(NULL, NULL, NULL);
+}
+
 SEC("tc")
-__failure __msg("combined stack size of 2 calls")
-int prog(struct __sk_buff *ctx)
+__failure __msg("combined stack size of 2 calls is 576. Too large")
+int pseudo_call_check(struct __sk_buff *ctx)
 {
 	struct hmap_elem *elem;
 	volatile char buf[256] = {};
@@ -37,4 +44,18 @@ int prog(struct __sk_buff *ctx)
 	return bpf_timer_set_callback(&elem->timer, timer_cb) + buf[0];
 }
 
+SEC("tc")
+__failure __msg("combined stack size of 2 calls is 608. Too large")
+int async_call_root_check(struct __sk_buff *ctx)
+{
+	struct hmap_elem *elem;
+	volatile char buf[256] = {};
+
+	elem = bpf_map_lookup_elem(&hmap, &(int){0});
+	if (!elem)
+		return 0;
+
+	return bpf_timer_set_callback(&elem->timer, bad_timer_cb) + buf[0];
+}
+
 char _license[] SEC("license") = "GPL";